1. Field of the Invention
The present invention relates to a charged particle generating apparatus for generating charged particles, and more particularly to a charged particle generating apparatus capable of preventing damage, which may be caused by incomplete contact between the cable connecting the electrode and the generating apparatus, to an electrode discharging charged particles.
2. Description of the Related Art
FIG. 1 shows a block diagram of a conventional charged particle generating apparatus. The charged particle generating apparatus 10 includes a first electrode 12, a second electrode 14, a current source 16, a first voltage source 18, a third electrode 20, a second voltage source 22 and a third voltage source 24. The charged particle generating apparatus 10 further includes a connection cable J1 and a connection cable J2. The connection cable J1 shown in FIG. 1 makes an electrical connection between the second electrode 14 and the voltage source 18. The connection cable J2 makes an electrical connection between the first electrode 12 and the current source 16. The charged particle generating apparatus 10 is an electron gun generating an electron beam 30.
The current source 16 supplies current to the first electrode 12 in order to heat the first electrode 12 to a predetermined temperature. The voltage source 18 then applies voltage to the second electrode 14 to generate an electric field between the first electrode 12 and the second electrode 14. The electric field is generated in such a way that the charged particles are attracted toward the second electrode 14. Thus, the electron beam 30 is discharged from the first electrode 12 to the second electrode 14 because of the attraction formed by the electric field generated by the second electrode 14. The second voltage source 22 applies negative voltage to the third electrode 20. The result of this is that the third electrode 20 electrically attracts the electron beam 30 toward the second electrode 14. The third voltage source 24 maintains the particle generating apparatus 10 with a negative potential to earth so that the electron beam 30 is easily discharged.
FIG. 2 (a) shows a current/time chart showing the ideal controlled current to be applied to the first electrode 12 for heating the first electrode 12 by the current source 16. If the maximum current Ih is applied to the first electrode 12 in one step, a large amount of current flows through the first electrode 12 instantaneously. This causes unequal heat distribution on the first electrode 12 and increases the tension in a part of the first electrode 12. The maximum current Ih is generally 2 to 3 amperes. The tension increase in the part of the first electrode 12 is undesirable because it may damage the first electrode 12. Therefore, the current is gradually applied to the first electrode 12 up to the maximum current Ih as shown in FIG. 2 (a). The time "t1" is on average from approximately ten seconds to one hundred seconds.
FIG. 2(b) shows a voltage/time chart showing the ideal controlled voltage to be applied to the second electrode 14 by the voltage source 18. If the maximum voltage Vmax is applied to the second electrode 14 in one step, the first electrode 12 may be damaged. The maximum voltage Vmax is generally about 5 kV to 7 kV. When the maximum voltage Vmax is applied to the second electrode 14, instantaneously a large amount of current may flow through the first electrode 12. This increases the tension in a part of the first electrode 12. Therefore, the voltage is gradually applied to the second electrode 14 up to the maximum voltage Vmax as shown in FIG. 2(b). The period between t1 and t2 is on average from approximately ten seconds to one hundred seconds.
The current/time characteristics and the voltage/time characteristics respectively shown in FIGS. 2(a) and 2(b) are predetermined by software included in the charged particle generating apparatus 10. Therefore, the conventional charged particle generating apparatus 10 cannot respond to an accident happening to any of the components included in the apparatus 10, for example, the current/time characteristic and the voltage/time characteristic when the connection cables J1 and J2 do not work normally.
FIG. 3(a) shows a current/time chart showing current applied at time t3 to the first electrode 12 by the current source 16 when the electric connection between these (the connection cable J2) is accidentally disconnected. The electric connection is then reformed at time t4. When the electric connection between the first electrode 12 and the current source 16 is disconnected at time t3, the current does not flow through the first electrode 12 thereby cooling it. When the electric connection between the first electrode 12 and the current source 16 is reformed at time t4, the maximum current Ih as shown in FIG. 2(a) is applied to the first electrode 12 instantaneously. As described above, if the maximum current is applied to the first electrode 12 in one step, the first electrode 12 may be damaged.
FIG. 3(b) shows a voltage/time chart showing voltage to be applied to the second electrode 14 by the voltage source 18 when the electric connection between these (the connection cable ii) is accidentally disconnected at time t5. The electric connection is then reformed at time t6. When the electric connection between the second electrode 14 and the voltage source 18 is disconnected at time t5, voltage is not supplied to the second electrode 14. When the electric connection between the second electrode 14 and the voltage source 18 is reformed at time t6, the maximum voltage Vmax as shown in FIG. 2(b) is instantaneously applied to the second electrode 14. As described above, if the maximum voltage is applied to the second electrode 14 in one step, a large amount of current may flow through the first electrode 12 and cause damage to the first electrode 12.